Whales filter feed with a tangled hair-like net

Bowhead whale under arctic ice

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Filter-feeding whales' unique baleen structures tangle to trap food, experiments have revealed.

US scientist Professor Alexander J Werth investigated how bowhead and humpback whales capture prey using the plates in their throats.

He found that, in flowing water, the fringed edges of the baleen tangle together to form a food-trapping net.

The study is published in the Journal of Experimental Biology.

"Bowhead whales are among the largest and most endangered of all whales," explained Prof Werth from Hampden-Sydney College, Virginia, US, who undertook the research.

"They feed primarily upon tiny copepods less than 1mm long, which they filter from the ocean with slow skim feeding."

To understand more about this feeding behaviour, Prof Werth investigated the unique baleen material responsible for filtering the whales' enormous gulps of water.

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Baleen plates are made of keratin: the same protein that makes hair and fingernails. The plates consist of two smooth layers with a third, fibrous layer sandwiched in the middle.

Whales have 300 plates of baleen inside their mouths, which are worn down at the edges by the animals' tongues to expose a bristly "fringe".

Prof Werth placed sample plates from both bowhead and humpback whales in a purpose-built flow tank, to test how they behaved in conditions similar to the wild.

While bowheads skim the water for microscopic prey, humpbacks perform long lunging dives to catch small fish. Although both whales have baleen filters, the bowheads' fringes are four times as long as their humpback cousins'.

Scientists tested how baleen behaves in flowing water.

"People presumed that baleen was simply a static material, but my work showed that it is a highly dynamic material whose porosity depends on the force and flow rate of the water moving through the whale's mouth," said the biologist.

"When I began testing plates of baleen in my circulating flow tank, I found that the fringes moved dramatically and became tangled together,"

The flow speed of the water and angle of the baleen had a marked effect on this trapping ability. Plates inside whales' mouths are perpendicular to the flowing water and Prof Werth found this to be the most effective position for capturing prey.

His experiments also revealed that, at the natural swimming speed for bowhead whales, the fringe on a single plate tangled to catch prey. But at faster speeds the hairs simply streamed through the water and the filter was effectively lost.

He then tested how multiple plates worked together to form a filter.

The fringe of a whale's baleen flowing in water Prof Werth used 20cm-long samples of baleen

"When I did these experiments I found that the multitude of fringes from all the plates got all tangled up and greatly decreased porosity, making it like a very finely meshed plankton net to catch tiny copepods and other very small planktonic creatures," Prof Werth told BBC Nature.

Despite their different feeding styles, Prof Werth found that the bowheads' and humpbacks' baleens performed similarly.

"This is an important finding because it shows how complicated the story of whale feeding truly is. There is so much that we have yet to learn about the biology of these huge creatures."

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